1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a laser printer, which utilizes electrostatic recording or electrophotography to develop an electrostatic image formed on an image bearing member by using developer (developing powder) made up of toner and carrier. More specifically, the present invention relates to monitoring and controlling of a toner density (a mixture ratio of toner to carrier) of developer in an image forming apparatus.
2. Description of the Related Art
Generally, in an electrophotographic image forming apparatus, an image is formed on a recording material, such as a sheet of paper, through various image forming processes of charging, exposure, development, transfer, fusing, and cleaning. More specifically, after uniformly charging the surface of an electrophotographic photosensitive member (hereinafter referred to as a “photosensitive member”), an electrostatic image (latent image) is formed by exposure corresponding to image information. The electrostatic image is developed into a toner image by using a toner, and the toner image is transferred from the photosensitive member to a recording material, e.g., a sheet of paper. After the transfer of the toner image, the photosensitive member is cleaned by removing the toner that remains on the surface of the photosensitive member after the transfer. On the other hand, the recording material including the transferred toner image is heated and pressed such that the toner image is fixed to the surface of the recording material. The image formation is thereby completed.
As a developer used in the above-described image forming apparatus, a two-component developer made of primarily a nonmagnetic toner and a magnetic carrier mixed with each other (hereinafter also referred to as a “two-component development method”) is widely utilized with a recent trend toward higher image quality and a higher speed of full-color image forming apparatuses. With the development method using the two-component developer, the developer including the toner and the carrier is supplied to the surface of a developer bearing member while the toner and the carrier are mixed with each other by a stirring and mixing member. A magnetic roll having a plurality of S and N poles alternately arranged thereon is fixedly positioned within the developer bearing member such that the developer comes into a spike-like standing state (hereinafter referred to as a “magnetic brush”) on the surface of the developer bearing member with the aid of magnetic forces generated by the poles. The toner is then attached to the electrostatic (latent) image by making the magnetic brush of the developer contacted with or positioned closely to the surface of the photosensitive member, and by applying a development bias voltage between the developer bearing member and the photosensitive member. As a result, the toner is attached to the electrostatic latent image and the development into the toner image is completed.
In a reversed development method, the development using the two-component developer is performed as follows. An electrostatic force is generated due to a potential difference (hereinafter referred to as a “development potential”) between an image area surface potential (V1 potential) on the photosensitive member and a development bias voltage (Vdc potential) applied to the developer bearing member. When the generated electrostatic force becomes larger than an electrostatic force acting to attach the toner and the carrier together, the toner is separated from the carrier and is attached onto the photosensitive member, thus performing the development. In a white blank area, a potential difference (hereinafter referred to as a “fogging prevention potential or Vback potential”) between a non-image area surface potential (Vd potential) on the photosensitive member and the development bias voltage (Vdc potential) is properly set so as to prevent the toner from attaching to the photosensitive member and to suppress toner fogging.
In a developing unit using the two-component developer, because a mixture ratio of the toner to the carrier (hereinafter referred to as a “toner density”) in the developing unit can change with consumption of the toner, the toner density needs to be monitored and maintained at an appropriate value. If the toner density is not maintained within an appropriate range, an improper mixture ratio of the toner density may cause an image failure, e.g., an image density variation, fogging, or carrier adhesion. Proper control of the toner density is hence important in order to form an image with high quality and high stability. Examples of a conventional method for controlling supply (replenishment) of the toner include a toner density detection method using a toner density detecting unit of the optical detection type or the inductance detection type, and a patch detection method (image density detection method).
Further, a tandem image forming process has recently been used with an increasing demand for a higher speed of the full-color image forming apparatus. With the tandem image forming process, a photosensitive member, a charging apparatus, an exposure apparatus, and a developing apparatus are provided for each of four colors, e.g., yellow, magenta, cyan and black. Those components are arranged in tandem such that an image is formed per unit including those components. By using the tandem image forming process, images of four colors can be formed at the same time and an image output speed can be increased.
Meanwhile, Japanese Patent Laid-Open No. 2004-117960 proposes a cleaner-less image forming apparatus in which a cleaning apparatus is omitted and a toner remaining on a photosensitive member after a transfer step is removed from the photosensitive member by “cleaning performed concurrently with development” in the developing apparatus so that the removed toner is recovered for reuse.
In the cleaning performed concurrently with the development, the toner remaining on the photosensitive member after the transfer step is recovered to the developing apparatus in the next or further subsequent developing step. More specifically, the photosensitive member including the toner remaining after the transfer step and attaching thereto is continuously charged and exposed to form an electrostatic latent image. In the step of developing the electrostatic latent image, of the toner remaining on the surface of the photosensitive member after the transfer step, the toner existing on an area not to be developed (i.e., an non-image area) is removed by application of the fogging prevention potential (Vback) and is recovered to the developing apparatus.
With the concurrent cleaning, because the toner remaining after the transfer step is recovered to the developing apparatus and is reused for the development of the electrostatic latent image in the next or further subsequent developing step, waste toner can be eliminated and maintenance operation can be simplified. Further, with the cleaner-less feature, the surface of the photosensitive member is not abraded by a cleaner. Accordingly, a surface film thickness of the photosensitive member can be kept constant and the lifetime of the photosensitive member can be prolonged. In addition, the cleaner-less feature is advantageous in reducing the size of the image forming apparatus.
In the cleaner-less image forming apparatus in which the cleaning is performed concurrently with the development, the following problem may occur when using, as the charging apparatus, a contact charging apparatus which is brought into contact with the photosensitive member to charge the surface of the photosensitive member. When the after-transfer remaining toner on the photosensitive member passes a contact nip (charging section) between the photosensitive member and the contact charging apparatus, a part of the after-transfer remaining toner, which has a charged polarity reversed to opposite one to a normal polarity, may attach to the contact charging apparatus. The attachment of such toner contaminates the contact charging apparatus beyond an allowable level and may cause a charging failure.
More specifically, the toner serving as a developer contains, though in a small amount, a toner which originally has a charged polarity reversed to opposite one to a normal polarity. Also, even with a toner having the normally charged polarity, the charged polarity of the toner may be reversed under the effect of a transfer bias or a separating discharge, for example, or the charge amount of the toner may be reduced with charge cancellation.
Therefore, the after-transfer remaining toner contains the toner having the normally charged polarity, the reversed toner having the opposite polarity, and the toner having the reduced charge amount in a mixed state. Of those kinds of toners, the reversed toner and the toner having the reduced charge amount may attach to the contact charging apparatus when they pass the contact nip (charging section) between the photosensitive member and the contact charging apparatus.
In order to remove and recover the toner remaining on the photosensitive member after the transfer step in the developing step, the after-transfer remaining toner carried to a developing section through the charging section is required to have the normally charged polarity and to have such a charge amount that the electrostatic latent image on the photosensitive member can be developed by the developing apparatus. The reversed toner and the toner having the improper charge amount cannot be removed and recovered from the photosensitive member to the developing apparatus, thus causing a failed image.
In order to prevent the toner from attaching to the contact charging apparatus, the following process may be performed. For example, the toner remaining on the photosensitive member after the transfer step, which is carried from the transfer section to the charging section and which contains variously charged toner particles (e.g., toner particles having the normally charged polarity, toner particles having the opposite polarity, and toner particles having reduced charge amounts), may be subjected to a charging operation so that the variously charged toner particles can be changed to toner particles having a normal polarity and uniform charge amounts.
Japanese Patent Laid-Open No. 2001-215798 and No. 2001-215799 disclose techniques to address the above-described problem. A toner charge-amount control unit for charging the after-transfer remaining toner is provided, as a charging auxiliary unit, upstream of a contact charging apparatus and downstream of a transfer unit in the moving direction of a photosensitive member. An after-transfer remaining toner uniformalizing unit (remaining toner uniformalizing unit) for making uniform the toner remaining on the photosensitive member after the transfer step is provided upstream of the toner charge-amount control unit and downstream of the transfer unit. The above-described problem may be overcome by contacting both the toner charge-amount control unit and the remaining toner uniformalizing unit with the surface of the photosensitive member and by applying constant DC voltages to those units.
More specifically, the toner remaining on the photosensitive member after the transfer step is uniformalized by the remaining toner uniformalizing unit, and the uniformalized after-transfer remaining toner on the photosensitive member is charged by the toner charge-amount control unit so as to have the normal polarity. Then, at the same time as charging the surface of the photosensitive member by the contact charging apparatus, the after-transfer remaining toner is charged by the toner charge-amount control unit so as to have the charge amount suitable for removing and recovering the after-transfer remaining toner by the developing apparatus through the cleaning performed concurrently with the development. As a result, the after-transfer remaining toner is recovered by the developing apparatus.
The problems that can occur when using the two-component development method will be described below.
If the carrier deteriorates with the long-term use of the image forming apparatus, the supplied toner may not be sufficiently charged in some cases. This causes the so-called fogging of the reversed toner, i.e., a phenomenon that the toner having the reversed polarity fogs over the photosensitive member. Because the reversed toner is charged so as to have the opposite polarity to that of the normal toner, the reversed toner is hardly transferred by the transfer unit and is recovered by the cleaning unit.
In the case of using the tandem image forming method, therefore, the following problem may arise. If the fogging occurs in one image forming unit on the downstream side in the moving direction of a transfer member, the fogging is transferred in a superimposed relation to a toner image formed by another image forming unit on the upstream side. Thus, a tint variation may be caused in an image finally formed on the transfer member.
In the case of using the cleaner-less method, because the cleaner blade is not provided, the reversed toner may contaminate the charging member and the charging auxiliary member if the fogging of the reversed toner occurs to a large extent. Such contamination may cause, e.g., undesired streaks in the image due to a charging failure.
To address the above-described problem, Japanese Patent Laid-Open No. 2003-316202 proposes a cleaner-less image forming apparatus in which toner contamination of a charging auxiliary brush is reduced by periodically expelling the toner out of the charging auxiliary member.
As one example of a method for detecting the occurrence of a fogging toner, Japanese Patent Laid-Open No. 9-281783 proposes a method of detecting the fogging toner by an optical sensor which is disposed on a photosensitive member.
Further, Japanese Patent Laid-Open No. 9-305009 proposes a technique of detecting an amount of toner attached to a magnetic brush charger from a current amount and correcting a charging condition.
With the apparatus proposed in Japanese Patent Laid-Open No. 2003-316202, however, the resulting effect is not sufficient because the apparatus does not intend to suppress the fogging of the reversed toner, which causes the contamination of the charging auxiliary member.
With the method proposed in Japanese Patent Laid-Open No. 9-281783, unless the toner fogging occurs to a large extent, detection accuracy is poor. It is hence difficult to detect a small amount of the fogging toner. Another disadvantage is that an additional space is required to dispose the optical sensor on the photosensitive member.
Further, the technique proposed in Japanese Patent Laid-Open No. 9-305009 does not discuss suppressing the fogging of the reversed toner, which causes the contamination of the charging auxiliary member. Another disadvantage is that one of two phenomena, i.e., whether the toner fogging occurs or a large amount of the after-transfer remaining toner occurs, cannot be discriminated just by detecting an amount of toner attached to the magnetic brush charger.